One of the most promising advanced systems for producing electrical energy from coal is the integrated gasification combined cycle since it can provide economically and environmentally sound systems which operate with improved thermal efficiency. When coal is gasified, however, sulfur that is present in most coals is converted to sulfur-bearing pollutant gases, in particular, hydrogen sulfide. In addition to environmental problems caused by these gases, protection of turbines and related equipment from corrosive action of sulfur-bearing gases also necessitates their removal.
Development of a sorbent in a form suitable for removal of hydrogen sulfide has been a challenging problem owing to requirements imposed for a successful operating system. The sorbent should be operational at high temperatures of 370.degree. to 760.degree. C. (700.degree. to 1,400.degree. F.) in the presence of reducing gases such as hydrogen and carbon monoxide. For obtaining effective contact with a gas stream, the sorbent should be incorporated in pellets suitable for use in a fixed, moving bed, or fluidized bed reactor. For successful operation of such a system, the pellets would of necessity be required to undergo repeated, alternating cycles of sulfidation and regeneration. In each cycle, a reactive oxide in the pellet is converted to a solid metal sulfide upon undergoing absorption of and reaction with hydrogen sulfide. After obtaining complete sulfidation, the spent pellet bed is then regenerated by being contacted with an oxidizing gas stream at a high temperature.
Various sorbents, mostly metal oxides, have been tested in the past for removal of hydrogen sulfide from a coal gasification stream. While these sorbents have shown a high initial reactivity, they have demonstrated deficiencies in their performance in pellet beds. Upon undergoing repeated cycles of sulfidation and regeneration, sorbent pellets have experienced either rapid and continuous decrease in reactivity or physical deterioration due to spalling during multi-cycle testing, making them undesirable. These deficiencies are believed to result from a decrease in surface area of a high-surface-area material in which the reactive oxide is incorporated, this decrease occurring as a consequence of prolonged exposure to high temperatures. Internal porosity is also decreased, the reactive oxide as a result becoming less available for further reaction. Paths in the pellets for passage of reactant gas tend to become closed off and ineffective. The pellets also may have insufficient porosity to provide internal space for the presence of metal sulfide or sulfate reaction products, resulting in cracking and spalling. Durable sorbent pellets that show a prolonged period of high reactivity with hydrogen sulfide along with resistance to spalling during repeated operating cycles of sulfidation and regeneration are therefore needed.